1. Field of the Invention
This invention relates to a method for characterizing solid surfaces by means of characteristic electromagnetic radiation emitted from atoms, ions, and molecules desorbed from the surface by means of incident radiation, and to methods for controlling materials processes by means of this surface characterization method.
2. Description of the Prior Art
The ability to characterize surfaces is a basic and essential requirement in many fields of technology, as well as in research. Many techniques for characterization exist, ranging from the relatively direct one of visual inspection by means of light microscopy to such relatively indirect ones as Auger spectroscopy. See for instance, Characterization of Solid Surfaces, P. F. Kane and G. B. Larrabee, editors, Plenum Press, New York and London, 1974, where some twenty surface characterization methods are reviewed.
All such methods have in common that a probe or stimulus is applied to the surface to be studied, and the result of the interaction of the stimulus with the surface is observed. A typical stimulus is radiation, where by "radiation" we mean in this context not just electromagnetic radiation but also corpuscular radiation, such as electrons, ions, atoms, molecules and the like. One of the basic sub-categories of these methods consists of the scattering methods, namely, those that detect and analyze the probe radiation after interaction with the surface. In this category are light microscopy, electron microscopy, X-ray scattering, and the like. Another basic sub-category consists of methods that detect and analyze radiation (other than probe radiation) originating from the surface or from the vicinity of the surface due to the interaction of the surface with the probe radiation. Methods using electron-beam excited X-rays, as for instance, the microbeam technique, X-ray fluorescence analysis, Electron Spectroscopy for Chemical Analysis (ESCA), Auger spectroscopy, and some forms of mass spectrometry are examples of methods belonging in this category. It is this latter category which is of interest in this application, and henceforth, our discussion will be restricted thereto.
A recently developed surface analysis technique utilizes the radiation emitted from excited particles sputtered from a solid surface by means of a low-energy ion or molecular beam. U.S. Pat. No. 3,644,044, issued Feb. 22, 1972, N. H. Tolk, C. W. White, "Method of Analyzing a Solid Surface From Photon Emissions of Sputtered Particles," and U.S. Pat. No. 3,767,925, issued Oct. 23, 1973 to E. B. Foley, Jr., et al, "Apparatus and Method for Determining the Spatial Distribution of Constituents and Contaminants of Solids." See also N. H. Tolk, I. S. E. Tsong, and C. W. White, "In Situ Spectrochemical Analysis of Solid Surfaces by Ion Beam Sputtering," Analytical Chemistry, Volume 49, pp. 16A-28A, January 1977. This method, referred to as SCANIIR (Surface Composition by Analysis of Neutral and Ion Impact Radiation) has evolved from recent experiments which showed that electromagnetic radiation, typically in the visible, ultraviolet, or infrared, is produced when beams of low-energy ions or neutral particles impact on a solid surface. Surface constituents can be determined by identification of the characteristic radiation emitted by the desorbed particles. This technique can be essentially nondestructive, since damage to the sample can be minimized by using low incident energies and low current densities. SCANIIR is a very useful analytical laboratory technique, but it is not being used to control or monitor manufacturing processes.
Although a plethora of spectroscopic surface analytic methods exists that rely on relatively low-energy electron probe beams, we will restrict outselves in this discussion of the prior art to those methods that rely on the observation of either an optical response from the sample, or that investigate the properties of particles desorbed from the surface by the probe beam.
The luminescence of surfaces bombarded with fast (100 keV) electrons has been observed some sixty years ago. Recently, it has been noticed that in the case of slow electrons the characteristics of the luminescence, i.e., intensity, polarization, and spectral distribution, are very sensitive to the condition of the surface, thus constituting a potentially useful method of surface characterization. See for instance, O. M. Artamonov and S. M. Samarin, "Radiative Interaction of Slow Electrons with Metal Surfaces," Radiation Effects, Volume 40, pp. 201-208 (1979). The luminescence spectra observed are invariably broadband, due to the fact that the radiation arises from excitations in the bulk of the material, albeit very close, i.e., typically within the order of a hundred Angstroms, of the surface. A. Shchurenko et al, "Light Radiation From Sodium Films Bombarded With Slow Electrons," Solid State Communications, Volume 33, pp. 141-142 (1980), for instance report optical radiation from Na films bombarded with slow electrons, and attribute the observed broad peak in the spectrum to radiation from surface plasmons. Fluorescence stimulated by low-energy electrons is, of course, not limited to metals. For instance, R. H. Prince et al, "Fluorescence of Ice by Low-Energy Electrons," The Journal of Chemical Physics, Volume 64(10), pp. 3978-3984 (1976) report the observation of the emission spectrum of solid H.sub.2 O during excitation by 200 eV electrons. The spectrum observed is broadband with some relatively broad peaks. No discrete narrow spectral lines were observed by these authors.
It has been known for some time that bombardment of surfaces with relatively low-energy electrons can lead to the desorption of particles from the surface, referred to in the literature as "Electron-Stimulated Desorption" (ESD). ESD has been observed for species chemisorbed or physisorbed at a solid surface, as well as for surface atoms intrinsic to the sample. A variety of inelastic interactions between the incident electrons and the surface can lead to ESD, although direct momentum exchange, typically the predominant mechanism in sputtering by ions or neutral atoms is not the dominant mechanism in ESD. This is, of course, due to the great disparity in mass between the incident electron and the desorbed particles, which have a mass ratio of at least approximately 1 to 2000. It is easy to show from classical collision theory that under these circumstances the energy transferred to the massive target particle in a direct collision is no more than about 4E.sub.i (m/M), where E.sub.i and m.sub.i are the energy and mass of the incident particle, respectively, and M is the mass of the desorbed particle. See for instance, T. E. Madey and J. T. Yates, Jr., "Electron-Stimulated Desorption as a Tool for Studies of Chemisorption: A Review," The Journal of Vacuum Science and Technology, Volume 8(4), pp. 525-555 (1971).
ESD can be studied either by methods based on changes in surface properties, or by methods based on the direct detection of desorbed particles. In the latter category are methods based on mass analysis, measurements of ion current and ion kinetic energy, and methods based on detection of secondary electrons. Ibid, also M. Szymonski et al, "Sputtering of Molecules During Low-Energy Electron Bombardment of Alkali Halides," Surface Science, Volume 90, pp. 274-279 (1979).
ESD as a technique for surface characterization by means of methods based on the direct detection of desorbed particles has a severe shortcoming. We are referring to the fact that detection and identification of desorbed particles requires either mass spectrometry or relatively complex indirect methods, all of these demanding equipment that is not typically available outside the laboratory, and that in any case is complicated and not conveniently used.
The discussion of the desorption of particles from surfaces has thus far been limited to desorption by means of electron beams. However, it has been recognized that photon beams can have a quite analogous effect, and that, therefore, irradiation of surfaces with appropriately chosen electromagnetic radiation will also lead to stimulated desorption of particles. See for instance, P. D. Townsend, "Photon-Induced Sputtering," Surface Science, Volume 90, pp. 256-264 (1979).